1. Field of the Invention
The invention relates to ink jet print heads. Specifically, the invention relates to the geometric arrangement of ink jet nozzles in the head.
2. Description of the Related Art
In on-demand ink jet printing, a grid of pixel locations is defined on a print media surface. During the print process, each pixel location may receive a droplet of ink from a set of ink ejection nozzles on a print head as the print head passes horizontally over the print media surface. In many systems, the pixel grid may be considered to comprise a series of vertical columns of pixel positions, and the ejection nozzles are also arranged in a vertical column. The vertical spacing between nozzles corresponds to the vertical pixel spacing, which will typically be approximately 50 to 600 pixels per inch, resulting in a vertical inter-nozzle spacing of about 40 to 500 microns. As the vertical column of nozzles passes over each vertical column of pixel locations, the appropriate droplets are deposited. Because the vertical column of nozzles is typically much shorter in length than the total number of pixels in a vertical pixel column for the whole image, the printer may sequentially pass the print head over one horizontally extending swath of the image at a time, incrementing the media between each pass. In some cases, multi-pass ink jet printing techniques using overlapping swaths are used to increase image quality.
It will be appreciated that printing an image will often require the firing of a large percentage or even all of the print head nozzles as the print head passes over a given vertical pixel column. In many print head designs, however, the simultaneous firing of too many nozzles, especially adjacent nozzles, is undesirable. In thermally activated print heads, for example, the firing of too many nozzles simultaneously results in a large power dissipation which is expensive to supply and which causes an excessive temperature increase in the print head. In addition, in both thermally and piezoelectrically actuated print heads, the firing of one or a set of nozzles may cause droplet volume and velocity changes or may otherwise interfere with the firing of other nozzles of the print head.
To help resolve these problems, nozzle arrangements have been developed in which the nozzles are not arranged precisely in a vertical column but instead deviate from each other horizontally. This horizontal deviation is much narrower than the horizontal pixel spacing, which is often, although not always, identical to the vertical pixel spacing. In one such embodiment, described in detail in U.S. Pat. No. 5,648,805 to Keefe, et al., the nozzles in a vertical column are arranged in 21 horizontally displaced sub-columns. Other thermal print head embodiments which have been designed include 13 sub-columns. In the 21 sub-column print head described in the ""805 patent, the vertical inter-nozzle spacing is about 85 microns, and each sub-column is horizontally displaced about 1.75 microns from the adjacent sub-columns. The total horizontal width of the 21 sub-columns is therefore about 36 microns. With a print head of this design, only one of the sub-columns is positioned directly over the center of a vertical pixel column at a time, and the nozzles for each sub-column are fired sequentially as each sub-column becomes properly positioned over the vertical pixel column. Thus, even if the vertical pixel column needs a deposited droplet on each pixel, only a few of the nozzles of the overall total need to be fired simultaneously.
Increasing the number of sub-columns reduces the maximum number of nozzles that must be fired simultaneously. However, as the number of sub-columns increases, the horizontal spacing between the sub-columns must decrease so that the first sub-column has not moved over the next vertical pixel column before the last sub-column deposits droplets onto the preceding vertical pixel column. A large number of sub-columns and the associated reduced sub-column spacing increases the complexity of the firing electronics, and may cause further nozzle cross talk issues due to the short time period between sub-column firing.
A drop-on-demand ink jet print head has a nozzle arrangement that reduces cross talk between nozzles with a minimal added complexity in firing electronics. In one embodiment, the invention comprises an ink jet print head comprising a column of ink ejection nozzles which is arranged in five to eight parallel sub-columns in such a way that no two vertically adjacent ink ejection nozzles are in either the same sub-column or are in two horizontally adjacent sub-columns. Parallel sub-columns may be spaced apart between approximately four and approximately 30 microns. Separation of nozzles of the column into sub-columns in this way provides a column of nozzles with a total width which is less than a print resolution unit of a printer the head will be used in, and also considerable cross talk reduction.
Ink jet printers including novel print heads are also provided. In one embodiment, an ink jet printer comprises a print head comprising a column of nozzles arranged in four to eight parallel sub-columns. The sub-columns are spaced apart such that the total width of the column of nozzles is less than one horizontal print resolution unit of the printer. In another embodiment, an ink jet printer comprises a platen forming a print surface, a media drive system configured to increment print media in a first direction over the print surface, and a movable print carriage configured to pass over the print media in a second direction perpendicular to the first direction between media drive system increments. The printer further comprises a piezoelectrically actuated drop-on-demand print head coupled to the moveable print carriage, wherein the drop-on-demand print head comprises one or more columns of nozzles extending in the first direction, each of which are arranged in five parallel sub-columns, wherein nozzle separation between the sub-columns in the second direction is approximately four to approximately thirty microns.